Calcium aluminates such as 3CaO.Al.sub.2 O.sub.3 ; CaO.Al.sub.2 O.sub.3 ; CaO.2Al.sub.2 O.sub.3 and CaO.6Al.sub.2 O.sub.3 and 12CaO.7Al.sub.2 O.sub.3 are extremely useful in many fields of metallurgy and in general as refractory materials.
Calcium aluminates are used in large amounts in the steel industry as a steel slag flux and as a desulfurizing agent for steel melts. For example, Schwer in U.S. Pat. No. 4,490,173 discloses a composition as an additive used in steelmaking. The proposed additive compositions include lime and a compound selected from a group comprising bauxite (which is the ore containing alumina) and mixtures of calcium aluminate and bauxite. Calcium aluminates are also important components in the preparation of cements, particularly aluminous cements.
Aluminum dross is formed whenever aluminum or an aluminum alloy is melted in air or other oxidizing atmosphere, and is thus obtained in large quantities in aluminum production and fabrication plants. Such dross is normally treated either by a plasma or in a conventional furnace with a salt mixture, to remove recoverable aluminum metal, leaving a dross residue having reduced aluminum content. The main components in dross residues include alumina, aluminum nitride, aluminum metallic, spinel (MgAl.sub.2 O.sub.4) and other minor components (Fe.sub.2 O.sub.3, SiO.sub.2, MgO). Dube et al. in U.S. Pat. Nos. 4,959,100 and 4,960,460 disclose treatment processes for recovering aluminum from aluminum dross hence producing the aluminum dross residue known in the trade as NOVAL.TM. (trademark of Alcan International Limited, Montreal, Canada). The composition of NOVAL.TM. brand aluminum dross residues is discussed in the '100 patent. Aluminum dross residues are frequently classified as hazardous material. Consequently, the disposal or transformation of the dross residues is of prime economic and environmental importance.
It is known that calcium aluminates may be obtained from aluminum dross or aluminum dross residues. For example, the possibility is well known in the industry to produce calcium aluminates by fusion of lime (calcium oxide CaO) and alumina (aluminum oxide Al.sub.2 O.sub.3). Gens discloses a process, in U.S. Pat. No. 5,135,565, for heating aluminum dross (not the residue) and lime at temperatures in excess of the dross slag melting point of approximately 1400.degree. C. to produce two main products, aluminum metal and calcium aluminate.
Lindsay et al. in U.S. Pat. No. 4,997,476 disclose a plasma process for recovering free aluminum from dross. The dross is charged in a plasma torch rotary furnace and is heated to its molten state. The free aluminum coalesces and is poured from the furnace. The resulting dross residue accumulates and lines the wall of the furnace. Periodically, the residue is scraped off and converted into useful non-metallic products.
Additionally there exist numerous methods, either chemical or thermochemical, which have been developed to synthesize calcium aluminates.
For example, Morozova et al. in Cement & Concrete Research. 1988, 18, 375-388 proposes the dissolution of calcium carbonate in an aqueous solution of aluminum chloride followed by a reaction with ammonium hydroxide to produce mixed Al-Ca hydroxides, which are dried and calcined to form CaO.Al.sub.2 O.sub.3, CaO.2Al.sub.2 O.sub.3, CaO.6Al.sub.2 O.sub.3 and 12CaO.7Al.sub.2 O.sub.3, which are used in high alumina cement.
In Chem. Papers, 1987, 41(6), 723-729, Nerad describes the preparation of 3CaO.Al.sub.2 O.sub.3 and 12CaO.7Al.sub.2 O.sub.3 by a precursor method, which consists in dissolving metallic aluminum and calcium carbonate in diluted nitric acid and reacting of the aluminum-calcium nitrates with tartaric acid, heating the resulting mixture, evaporating the liquids, and calcining the remaining solid to give the above calcium aluminates.
Other processes teach compositions used as additives for steelmaking and iron foundries. Fiodorov et al. (Lensovet Technological Institute, Leningrad) produce CaO.Al.sub.2 O, 12CaO.7Al.sub.2 O.sub.3 and 3CaO.Al.sub.2 O.sub.3 by immersing pellets of limestone and alumina into molten pig iron. The synthesis of the aluminates is said to be achieved within 30 and 2000 seconds. The temperature of the reactions varies from 1300.degree.-1500.degree. C. The nature of the end products is a function of the composition of the mixture of raw materials, which is adjusted to suit high-alumina cements.
Craig et al. in U.S. Pat. No. 4,941,914 disclose a desulfurizing additive for molten pig iron. The additive is comprised mainly of calcium carbide and a minor admixture of a calcium aluminate slag. The calcium carbide is said to act as a mechanical binding agent in the additive mixture. Optionally, asphalt is proposed as providing better cohesion to the additive mixture. Interestingly, the patent mentions that the calcium aluminate slag can be prepared by heating and melting a mixture of 50 to 65% lime, 25 to 35% alumina, and 0 to 10% calcium fluoride. It is to be noted that the melting point of such mixture is reported in the prior art as being approximately 1400.degree. to 2000.degree. C. (FIG. 231, "Metal Oxide Systems" page 102).
Furthermore a grave technical difficulty arises when aluminum dross residue is charged in a furnace. Dust and other small particles tend to fly out of the furnace consequently failing to convert to calcium aluminates and generally causing nuisance and pollution.
For these reasons, the production of calcium aluminate slag by a direct melting heat treatment of alumina and lime or limestone or other additives is presently not cost effective.
Accordingly, there is a substantial need in the industry for an improved process for producing calcium aluminates from aluminum dross residue such as NOVAL.TM. brand aluminum dross residues. It is particularly desirable that such process be cost effective and environmentally responsible.